1. Field of the Invention
The invention relates to apparatus and methods for assay of metals, especially for assessing the purity of precious metals and alloys. A meter circuit coupled to an inventive probe measures the extent of galvanic action of dissimilar metals in the presence of an electrolyte, one of the metals being the sample to be assayed. The invention is useful for testing the metal content of coins, art objects jewelry, and the like, in that the probe can simply be touched against the sample to provide a reading. The reading is developed by measuring the extent to which a biasing voltage is reduced by galvanic action between the sample and a preferably platinum probe conductor embedded in a fibrous absorbent medium containing an electrolyte.
2. Prior Art
There are a number of analytical techniques used to assess the nature of metal samples, for example to determine the precious metal content of coins, art objects and items of jewelry. A traditional test used by jewelers and precious metals traders to determine the purity of gold alloys, for example, relies on a set of "gold pencils" that are used in comparing the results of a scratch test. In this method, the jeweler has a number gold alloy "pencils" or rods used as references. The gold alloy pencils can be graduated, for example, over a range of 4 to 24 karat gold alloy compositions.
The reference gold alloy pencils are used to mark an abrasive stone or touchstone, for comparison with a mark made using the sample. The jeweler makes a small scratch using the specimen to be analyzed, and reference scratches with the gold alloy rods or pencils, in each case leaving a trace quantity of metal. The jeweler then places an acid such as nitric acid or aqua regia (an aqueous solution of hydrochloric and nitric acids) on the abrasive stone and compares the color of the traces produced by the specimen scratch with the color of the traces of the several gold alloy rod scratches, to assess the karat value of the sample.
An experienced jeweler can estimate the amount of gold in karats in the specimen being analyzed by comparing the colors of the sample trace and reference traces. This method, which is still widely used, is dependent on the skill and experience of the jeweler employing the test. Such testing is time consuming and causes some damage to the sample. Moreover, the results can turn on subjective judgment. Errors made in technique or in judgment in this field, however, can be costly, either to the customer or to the jeweler.
More recently, attempts have been made to develop a more objective assaying technique using electrical testing apparatus. One technique is to measure the electromotive force (EMF) developed as a result of galvanic action when the specimen to be analyzed is brought into conductive contact with a dissimilar metal. This is accomplished in the presence of an electrolyte. The sample and the dissimilar metal electrode produce a galvanic voltage in the manner of a very low power battery, which can be measured to assess the nature of the sample. Several of these prior apparatus and methods are described in the following United States patents:
______________________________________ No. 2,531,747 Stearn 1950 No. 3,282,804 Stearn 1966 No. 4,376,027 Smith 1983 No. 4,799,999 Medvinsky 1989 No. 5,080,766 Moment 1992 No. 5,218,303 Medvinsky 1993 ______________________________________
Whereas the electromotive force produced is relatively small and the differences in electromotive force to be measured between nearly comparable samples is even smaller (e.g., 20 karat vs. 22 karat gold), the accuracy of measurement turns on the precise manner in which the sample, electrolyte and electrode come into contact and interact. Insofar as the apparatus and methods described in these prior patents require the judgment and skill of an experienced user for setup and use, they could advantageously be improved. Furthermore, it would be advantageous to provide an objective form of test that is more convenient, does not stain or damage the specimen, and produces accurate results repeatably and reliably without a great deal of setup and adjustment.
The commercially available apparatus of the type described in U.S. Pat. Nos. 4,799,999 and 5,218,303 to Medvinsky are such that a quantity of electrolyte is applied to the sample for each test, for example being discharged from a syringe-like means. An electrode is brought into contact with the electrolyte and into proximity with the sample, without shorting directly against the sample, thereby making a so-called "wet junction." A gelatinous or viscous electrolyte or an electrolyte paste can be used. In other arrangements, the sample is dipped into a container of electrolyte.
When coupled into a circuit, galvanization anodizes the sample. As a result these procedures can leave a stain on the specimen. A stain of this type typically must be removed from the specimen after the test, for example with an abrasive that removes specimen material in a manner not unlike the use of a touchstone. In addition, such apparatus is not convenient for allowing tests to be performed quickly on several specimens in rapid succession, because a fresh supply of the gel or paste electrolyte must be dispensed for each test.
The voltage reading obtained using the test can be affected by the availability of free ions in the electrolyte and other factors. According to Medvinsky, an exciting voltage is applied to the galvanic circuit including the specimen for five seconds and then released, and the decaying galvanic voltage is measured after a predetermined time (e.g., five more seconds). Each test thus requires at least ten seconds and the accuracy of the results is dependent on timing. The galvanic potential achieved and the decay rate must be measured accurately. The results are compared to standards stored in the memory of the electronic apparatus or manually by comparison with tables developed by testing reference samples of known purity.
For repeatable results in galvanic testing, the test conditions must be identical from test to test. Thus the surfaces of the specimen(s) and the electrode, and their interaction with the electrolyte, cannot be materially different. This is difficult to achieve. The electrode tip of the prior commercial apparatus described in U.S. Pat. Nos. 4,799,999 and 5,218,303 requires frequent cleaning to remove dried and/or spent electrolyte paste from the tip. The electrolyte gel or paste, which comprises hydrochloric acid, can dry out between tests, which affects the repeatability of results. The electrolyte also can become too liquified and flow out of the dispenser, which can be avoided if the electrolyte is refrigerated. It would be advantageous if an electrolyte could be deployed in a more convenient manner, while still obtaining accurate and repeatable results.
Other apparatus are similarly inconvenient. For example, the technique described in U.S. Pat. No. 5,080,766--Moment requires that the entire specimen be immersed in a container filled with electrolyte. This test also requires considerable waiting time to obtain a measurement of the galvanic electromotive forces being developed.
Further problems relate to the specific circuitry used to measure the galvanic action, and typically to provide a reference voltage level that is coupled to the galvanic circuit. It is desirable to obtain a reasonable measurement span in order to obtain good resolution, ideally to distinguish down to a karat or fraction of a karat. This is also difficult to achieve, particularly at high karat levels, because the slope of a curve of galvanic action vs. karat level or purity, drops off at higher karat levels.